This invention relates to methods of fixing images to a transfer paper, specifically where the images are comprised of marking particles suspended in an oil carrier, and more specifically where the images are comprised of photosensitive pigments suspended in an oil carrier, as in photoelectrophoretic inks.
In the photoelectrophoretic imaging process, an image is formed from an imaging suspension or ink by subjecting the ink to an electric field and exposing it to activating electromagnetic radiation, e.g., visible light. The imaging suspension is comprised of light sensitive particles suspended within an insulating liquid carrier and believed to bear a net electrical charge while in suspension. Normally, the ink is placed between injecting and blocking electrodes used to establish the electric field and is exposed to a light image through one of the electrodes which is at least partially transparent. According to one theory, particles attracted to the injecting electrode by the electric field exchange charge with the injecting electrode when exposed to light and migrate under the influence of the field through the liquid carrier to the blocking electrode. As a result of the migration, positive and negative images are formed on the two electrodes. The blocking electrode is covered with a dielectric material to prevent charge exchange with the particles and thereby prevent the particles from oscillating back and forth between the two electrodes.
The photoelectrophoretic imaging process is either monochromatic or polychromatic depending upon whether the light sensitive particles within the liquid carrier are responsive to the same or different portions of the light spectrum. A full color polychromatic system is obtained, for example, by using cyan, magenta and yellow colored particles which are responsive to red, green, and blue light respectively. An extensive and detailed description of the photoelectrophoretic process is found in U.S. Pat. Nos. 3,384,565 and 3,384,484 to Tulagin and Carreira, 3,383,993 to Yeh and 3,384,566 to Clark, and the disclosures of these patents are expressly incorporated by reference into the present disclosure.
Following particle migration and image formation on the electrodes, the base, or injecting, electrode with the positive image thereon is separated from the blocking electrode. Where the positive is to be transferred, as it most often will be, the base electrode is contacted with the transfer member in some suitable manner, as described, for example, in U.S. Pat. Nos. 3,565,614; 3,642,364; and 3,705,797. The next step usually is to fix the transferred image to the transfer sheet, or if the image is to remain on the injecting electrode, to fix the image to the electrode. Generally, images formed by particles such as suspended pigments are fixed by somehow bonding the particles to the substrate material. Photoelectrophoretic ink images have, in the past, usually been fixed by thermoplastic bonding; i.e., the pigment particles are bound by softened thermoplastic materials, which upon cooling, re-harden and trap the pigments on the image substrate. The cited patents mention several methods for accomplishing the bonding. For example, a thermoplastic adhesive can be coated on the transfer member, softened, and contacted with the imaged electrode; the image is then pulled onto the transfer member, and the plastic allowed to cool to trap and bond the image particles. The transferred image or electrode image may alternatively be laminated with a thermoplastic material. Yet another approach involves inclusion of the bonding agent in the ink itself, which upon heating, traps the image particles in-situ.
However, all of the above transfer and fixing methods function optimally only when the insulating carrier for the marking particles is allowed to volatilize, or is otherwise disposed of. Where the carrier is a basically non-volatile material, such as an oil, it will interfere with the fixing process, particularly one which is designed to operate at higher speeds, as in a continuous belt or web system. In this instance, the process must operate slowly enough to allow the oil to be absorbed into the substrate or transfer paper. Otherwise, the oil will "bleed" out of the thermoplastic bond and result in an oily copy. Additional problems are encountered where the transfer member is pre-coated with a thermoplastic, in which case the oil will have to permeate the material before the image can be satisfactorily bonded. Additionally, absorption of the oil into the transfer sheet frequently results in spotting, or transparentizing, of the sheet. The same deficiencies are present when other tackifiable bonding materials are used, namely, dispose of the non-volatile carrier.
At first blush, it would seem that one solution to the problem would be use of an oil-absorbing material on the transfer sheet. The problem, however, still exists of fixing the pigment particles after the oil has been taken care of. For example, U.S. Pat. No. 3,811,914 teaches in part the selective incorporation of oil-absorbing materials, such as starch and clay, into transfer papers to adjust the oil absorption properties of the paper. The object therein is to minimize oil takeup by the paper without impeding toner particle takeup by the paper fibers. However, the inventors have found that even dense, wellformed images held by the presence of such absorption agents are still subject to damage in handling due to damage of the relatively frangible oil-absorbing materials.